The basic operational concepts of the internal combustion engine have remained largely unchanged for much of the past 130 years, since the patent issued to Karl Benz in 1886. Only relatively recently have developments in sensors and computer technology allowed rapid advances in combustion engine efficiency. The overall efficiency of the internal combustion engine driven vehicle, calculated across the entire energy chain “from oil well to wheels” is currently around 15%. Occupying a key position in this energy chain, the efficiency of a 4-stroke gasoline engine, the type of motor vehicle engine in most widespread use, is around 30%; for diesel fueled 4-stroke engines the efficiency is around 40%. These are laboratory efficiency numbers for engines operating at design capacity. In practice, especially for large engines operating under partial loads, the efficiencies are significantly lower, as evidenced by the low MPG numbers that characterize normally aspirated (i.e. non turbo-charged) gasoline sports cars. These cars are relatively lightweight, with large capacity engines, and parallel exhaust systems. Nevertheless, the efficiencies realized by such cars are very low.
Any improvement in the efficiency of four-stroke engines is highly desirable, on the grounds of direct and indirect costs to the user and the environment.
Engine efficiency may be improved by addressing input or output aspects of the combustion process. Direct fuel injection and variable input valve timing are well known and well developed examples of the former. The current invention addresses the output aspect, proposing a novel method of residual exhaust gas management.
Ideally, at the end of the exhaust stroke of a 4-stroke engine, all the gas in the cylinder space would be completely expelled. This may indeed happen in the engines of race cars, where the exhaust port of each cylinder voids to the outside air in a very short distance, without having to pass through the restrictions of a muffler or catalytic converter. Also, as these cars run at very high speeds, significant vacuum suction is generated at the exhaust port, which contributes to drawing virtually all the gas out of the cylinder at the end of each exhaust stroke.
However, in a standard transportation vehicle, whether fueled by diesel or gasoline, the exhaust pathway is not so unrestrained. It typically includes a muffler to reduce emitted noise, and a catalytic converter to convert unburned CO, hydrocarbons, and NO(x) to CO2, H2O, and Nitrogen. While they provide valuable protective functions, these introduced devices act to restrict the outflow of exhaust gases, creating significant back pressure at the cylinder exhaust port. One current manufacturer of diesel engines actually adds ammonia into the exhaust pathway to react with particulate matter in the exhaust stream and thus avoid emission of that matter into the environment. All such attempts to reduce the emitted pollution have the unfortunate effect of increasing back pressure, which, as indicated in FIG. 1, prevents all the gas from being expelled at the end of the exhaust stroke. The residual oxygen-depleted gas in effect dilutes the air taken in at the next intake stroke, as shown in FIG. 2. This in turn obviously reduces efficiency from the value it could theoretically achieve if all of the gas mixture to be compressed and combusted were freshly input oxygen-containing air.
A more detailed description of the relevant processes is given in the “Detailed Description of the Invention” section following, but as an illustrative quantitative example, consider a typical cylinder compression ratio of 10:1 and an “ideal” back pressure of 1 Atm. In this case, there will be a volumetric dilution of about 10% causing a corresponding lowering of the engine efficiency relative to its potential ideal of 10%. Higher back pressures causing greater dilution will result in correspondingly greater drops in efficiency. For simplicity, this analysis ignores the complication that gas densities drop as temperature rises, so at a typical high exhaust temperature of around 700 degrees C., the density of the residual gas may be only 50% of the density of the input gas at room temperature. However, a typical back pressure is likely to be closer to 2 Atm than 1 Atm, and in the case of diesel engines, back pressures of 3 Atm or even 4 Atm have been reported. Therefore, the simplified estimates above regarding dilution and efficiency are not only reasonable but conservative.
One current approach to improving the outflow of exhaust gas is to reduce back pressure by providing an exhaust port header that creates a cancelling back pressure by collecting the output from all the cylinders in a 4:1 junction and reflecting a “combined” pressure back to the exhaust ports. This type of approach is not only complicated and costly, but also can only work well for an engine running at a fixed rpm value, actually increasing back pressure at other values. It may be advantageous for airplane engines, which do effectively operate at a fixed rpm, but is not really useful for standard variable rpm car engines. Another current approach provides multiple parallel exhaust pathways (e.g. dual or quadruple exhaust systems). This incurs significant penalties in weight, cost, and size.
The trend towards increasingly stringent anti-pollution requirements is likely to exacerbate the problem of exhaust back pressure lowering car engine efficiency. What is needed is a method of improving the outflow of residual oxygen-depleted gases from the cylinders without adding heavy components or interfering with the functions of devices in the exhaust system that reduce emitted noise and pollutants.
It should be noted that the current application is directed to four-stroke engines, not two-stroke ones. The latter use a process called scavenging to flush out exhaust gas, introducing a fresh charge of fuel/air mixture mid-way through the power stroke. The inflow of this mixture forces spent, combusted gas out through the exhaust port but also inevitably forces some non-combusted fuel out as well as a significant amount of burned lubrication oil. The polluting problems of such engines are obviously great, and are not addressed by the proposals disclosed in this application.